The invention relates to an electronic circuit for operating a High Intensity Discharge (HID) lamp, in particular an Ultra High Pressure (UHP) lamp.
The invention further relates to an image projector with the electronic circuit for operating a High Intensity Discharge (HID) lamp, in particular an Ultra High Pressure (UHP) lamp.
HID and UHP lamps are known in principle from the prior art. They are preferably used for projection purposes, but also, for example, for operating automobile headlights. Their features are a very small light arc accompanied by a high luminous efficacy, which leads to a very good overall efficiency. The brightness of these lamps is approximately two to four times that of other gas discharge lamps.
A disadvantage of these HID lamps, however, is the effect of arc shift, i.e., a change in the position of the light arc during the operation of these lamps. The proportion of the total amount of light generated by the lamp entering the image-generating system is changed by the change in arc position, so that the brightness of the projected image fluctuates. This effect also leads to undesirable fluctuations in the brightness distribution on the image generator. A flicker effect observable to the viewer is the result.
Various measures are known from the prior art for reducing this flicker effect.
A first measure is to provide an additional high current pulse in the waveform of the lamp current before the commutation thereof. This special shape of the lamp current is capable of suppressing the arc shift and thus the flicker effect successfully.
The provision of the high current pulse, however, has the disadvantage that the lamp ballast becomes larger and more expensive than for a lamp current of different shape, and also that the operational life of the HID lamp is clearly reduced.
A second measure which also may be suitable for reducing the flicker effect is disclosed in JP-2000028988A and is shown in FIG. 3. The JP document does primarily describe the solution to another problem, i.e., a gradual change in the lamp brightness over its total life, but it also discloses, though not explicitly, those criteria which must be fulfilled for a suppression of the flicker effect. Those skilled in the art will indeed derive suitable measures for reducing the flicker effect from the JP document at least indirectly. Referring to FIG. 3, JP-200028988A discloses an LCD projector with an optical system 420 and an electric circuit. The optical system 420 comprises a gas discharge lamp 422 with a reflector 421 and an integrator 423 connected downstream of the lamp, an image generator 424, and an objective 425b. The integrator 423 together with a condenser 425a safeguards a homogeneous brightness distribution in the illumination of the image generator 424, and thus in the image generated by the image generator 424. The electric circuit serves to operate the lamp 422. To this end, the electric circuit employs a lamp ballast 410 for offering a controlled lamp current to the lamp 422 in response to a control signal, and a brightness sensor 430 for generating and issuing a sensor signal. The sensor signal here represents the quantity of light given off by the lamp 422 at the location of the brightness sensor 430. The quantity of light represented by the sensor signal is compared with a given reference quantity of light in a microprocessor 440 so as to generate the control signal in dependence on the measured light quantity deviation and to provide it to the lamp ballast 410. The generated quantity of light is thus controlled to the reference value. A flicker effect may be prevented if the light quantity control takes place quickly enough.
The quantity of light given off by gas discharge lamps at a constant power decreases in the course of lamp life owing to various causes. To have a possibility of safeguarding nevertheless a constant brightness over the entire lamp life, it is suggested in the cited Japanese publication JP-2000028988A to operate the lamp at a power substantially below its rated power at the start of lamp life and to increase the operational power as lamp life progresses so as to obtain a constant brightness of the light generated by the lamp. This, however, is only possible until the moment the rated power is reached.
This second measure, however, has a drawback. Specifically, since the lamp is initially operated at a power below the rated power, the generated brightness is substantially lower than in the case of rated power, i.e., such a projector system requires a bigger lamp for generating the same brightness right from the start than does a system without this kind of control.
HID lamps, moreover, are characterized by a sensitive thermal balance which can be maintained satisfactorily at rated power only. Adverse effects in lamp life are to be expected in the case of deviations, so that the control in the manner of JP-2000028988A leads us to expect a shortened lamp life.
Furthermore, a positive and negative control of the brightness is possible at the start only. This possibility becomes smaller as the operating power rises and finally disappears entirely when the lamp is operated at its rated power. It should finally be noted that sensor defects in the disclosed circuit, for example an erroneous internal sensor gain factor, will immediately lead to an erroneous control signal and thus to an undesirable control behavior. The disclosed circuit thus as a rule requires particularly expensive and complicated sensors so as to avoid sensor errors.
In a particular, the brightness sensor in the known circuit should operate reliably not only at room temperature, but also at high temperatures prevailing inside an image projector.
Given this prior art, it is an object of the present invention to develop an electronic circuit for operating a HID lamp and an image projector with such an electronic circuit further such that a control of the brightness is rendered possible throughout lamp life and sensors of simpler construction, and thus less expensive sensors can be used.
This object is achieved by an electric circuit employing a high-pass filter for offering the control signal through high-pass filtering of the sensor signal.
Very low-frequency components of the brightness fluctuations, and in particular the DC component thereof, are filtered out from the sensor signal by the high-pass filter. These frequency components will thus be absent also in the control signal and will not be involved in the control of the HID lamp.
The remaining AC components of the brightness fluctuations are controlled down to zero, according to the invention, instead of controlling the absolute brightness to a given reference value, as in the prior art.
This has the advantage on the one hand that influences of erroneous offsets or erroneous sensitivities of the brightness sensor are filtered out from the sensor signal and thus exert no undesirable influence on the control. It is accordingly very well possible to use simple, inexpensive sensors for realizing the circuit according to the invention without the quality of the control being impaired thereby.
On the other hand, the high-pass filter advantageously allows an elimination by the control circuit of the brightness fluctuations throughout the entire life of the HID lamp. The elimination by the control circuit according to the invention is possible both in a positive direction and in a negative direction also during operation at rated power.
The flicker effect is effectively suppressed for the human eye in the control of the lamp according to the invention.
In an advantageous embodiment, a control unit within the lamp ballast is furthermore designed for controlling the electric power generated at the output of the control unit such that the HID lamp is operated constantly at its rated power level for a long period. As a result, the lamp life is maximized, while on the other hand it is safeguarded that the light output of the lamp is a maximum throughout its entire life.
The object of the invention is furthermore achieved an image projector incorporating the advantages mentioned above with reference to the electronic circuit. The human eye is particularly sensitive to flicker effects in the representation of still images with large, monochrome surfaces. The suppression of this effect is accordingly particularly advantageous here.